1. Field of the Invention
The invention relates to a gas purifying catalyst, and, more particularly, to a burned gas purifying catalyst for use with an exhaust system of an automobile engine suitable for emission level controls of nitrogen oxides (NOx), hydrocarbons (HC) and carbon monoxide (CO).
2. Description of the Related Art
As one of catalysts installed in an exhaust line of an automobile engine to purify the exhaust gas or to significantly lower emission levels of oxides of nitrogen (NOx) as well as hydrocarbons (HC) and carbon monoxide (CO) in the exhaust gas an automobile exhaust control catalyst, there has been a monolith type catalytic convertor which is formed with an under catalyst layer having active alumina particles and platinum (Pt) and rhodium (Rh) on a monolith honeycomb carrier and an over catalyst layer, coated over the under catalyst layer, which comprises barium-fixed ceria (cerium oxide) particles, active alumina particles and palladium (Pd). Such a catalyst is known from, for instance, Japanese Unexamined Patent Publication No. 3-207446. The reason for fixing barium (Ba) to the ceria particles is to prevent the ceria from suffering heat deterioration. The barium-fixed ceria particles are produced in such a manner to dry and solidify a mixture of a barium nitrate solution with ceria particles as a solid lump of barium nitrate-adsorbed ceria particles and break it into particles. The over catalyst layer is coated by dipping a catalyst carrier in a slurry of a palladium chloride solution with the barium nitrate-adsorbed ceria particles and, thereafter, dry and burn the slurry film on the catalyst carrier.
While barium (Ba) is essentially intended to play as an agent to prevent heat deterioration of ceria, it is in some cases used to purify exhaust gases, in particular to reduce nitrogen oxides (NOx) in exhaust gases as is known from, for instance, Japanese Unexamined Patent Publication No. 7-108172. The catalyst described in the Japanese Unexamined Patent Publication No. 7-108172 is a monolith honeycomb type catalytic convertor that carries an under catalyst layer having barium (Ba) supported by an alumina support and an over catalyst layer having platinum (Pt) and rhodium (Rh) supported by an alumina support. This catalyst reduces nitrogen oxides (NOx) through the steps of oxidizing nitrogen oxides (NOx) with the barium (Ba) in the over catalyst layer, a lowering the concentration of oxygen (O2) in the exhaust gas so as to produce a reducing atmosphere in which the nitrogen oxides (NOx) are separated from the barium, and reducing the nitrogen oxides (NOx) by the catalytic metal in the under catalyst layer making the utilization of hydrocarbons (HC) and carbon monoxide (CO) in the exhaust gas as reducing agents.
A typical problem the exhaust gas emission control catalysts experience is sulfur (S) poisoning and/or water (H2O) poisoning and is significant in particular if they contain large amounts of barium which has a strong tendency to be poisoned. It has been proved by the inventors of this invention that lanthanum (La) has a tendency of the sulfur (S) poisoning and/or water (H2O) poisoning as well. Accordingly, unless the catalyst containing platinum and rhodium or palladium as main catalytic metals is kept away from the sulfur (S) poisoning and water (H2O) poisoning, it is difficult that the catalyst maintains its intended emission control efficiency for a long period of time.
It is an object of the present invention to provide a catalyst construction for purifying gases which prevents lanthanum and barium from sulfur poisoning and/or water poisoning.
It is another object of the present invention to provide an catalyst construction for purifying automobile exhaust gases which maintains its intended emission control efficiency for a long period of time.
This invention has been achieved on the basis of the knowledge obtained from the results of various investigations and assessment conducted by the inventors of this application that a combination of a composition of barium and lanthanum and a zeolite support prevents the barium and lanthanum from sulfur poisoning and/or water poisoning and maintains the intended activity of the barium and lanthanum for a long period of time.
These objects of the present invention are achieved by providing a gas purifying catalyst construction comprising an under catalyst layer containing at least one of barium and lanthanum and an over catalyst layer containing an agent to absorbing water in a gas. The over catalyst layer prevents the barium and/or the lanthanum from sulfur poisoning and/or water poisoning. The catalytic metal may be contained either one or both of the under and over catalytic layers.
The water adsorbing agent comprises a crystalline metal silicate which works to prevent the barium and lanthanum from sulfur poisoning and/or water poisoning. This is because the metal silicate, such as MFI-type zeolite, is one of materials that exhibit excellent water adsorbing performance and prevent water poisoning, consequently. Further, the metal silicate in the over catalyst layer prevents the barium and lanthanum in the under catalyst layer from easily contacting with gases, enhancing the prevention of sulfur poisoning and/or water poisoning.
The over catalyst layer contains one or more selected from a noble metal group of catalytic materials such as platinum (Pt), rhodium (Rh), palladium (Pd) and iridium (Ir), which works to lower emission levels of oxides of nitrogen (NOx) as well as hydrocarbons (HC) and carbon monoxide (CO) in burned gases. In particular, when the over catalyst layer contains platinum and rhodium as the noble metal group of catalytic materials, the catalyst exhibits an excellent NOx emission control efficiency in burned gas resulting from the combustion of a lean air-fuel mixture through a synergistic effect of the platinum and rhodium catalytic materials and additives such as barium or lanthanum in combination.
If a small amount of palladium is added into the under catalyst layer, it is preferred to support the palladium by a cerium oxide or alumina so as to deposit the palladium particles separately from the rhodium particles. The reason for a decline in the catalytic activity with a rise in ambient temperature has been considered to be caused by the absence of intermediate products of the hydrocarbon combustion contributory to reduction or decomposition of nitrogen oxides which results from expeditious combustion of hydrocarbons. Although the reason for the improvement in high temperature NOx emission control efficiency of the catalyst resulting from the presence of the cerium oxide has not been clearly proved, the presence of cerium in the catalyst of the invention suppresses the combustion of hydrocarbons at high temperatures and produces easily intermediate products of the hydrocarbon combustion. When the cerium is contained in the under catalyst layer, the cerium is prevented by the barium and lanthanum from experiencing thermal deterioration and exhibits its primary chemical activity for a long period of time.
The under catalyst layer may contain an additive of palladium or alumina with the effect of improving low temperature catalytic activity of the catalyst. The palladium exhibits its catalytic activity at temperatures lower as compared with platinum and rhodium and burns hydrocarbons in low temperature exhaust gases from an automobile engine which is still cold. Consequently, even when the exhaust gas is still at low temperatures, the combustion of hydrocarbons by means of the under catalyst layer causes the over catalyst layer to rapidly raise its temperature sufficiently to burn hydrocarbons. The combustion of hydrocarbons is utilized to reduce or decompose nitrogen oxides in burned gases.
Contact of the palladium in the under catalyst layer with a large amount of hydrocarbons causes poisoning, lowering the catalytic performance. However, the metal silicate contained in the over catalyst layer absorbs hydrocarbons in burned gases and, consequently, prevent the palladium in the under catalyst layer from hydrocarbon poisoning even when insufficient combustion of hydrocarbons occurs while the engine is still cold.
Because, unlike platinum and rhodium, palladium is easy to exhibit its catalytic activity rather after having been oxidized, the catalyst containing palladium and alumina in the under catalyst layer exhibits well its catalytic activity since the alumina works more easily as a source of oxygen supply and promotes oxidization of the palladium. In this instance, the alumina is prevented by the barium and lanthanum from thermal deterioration.
The over catalyst layer preferably has a weight proportion relative to the total weight of the over and under catalyst layers in a range between {fraction (3/40)} and {fraction (34/40)}. If the lower limit is exceeded, it will be difficult for the catalyst to form the over catalyst completely over the under catalyst layer and to exhibit an intended NOx emission control efficiency. On the other hand, if the upper limit is exceeded, the over catalyst layer arrests the catalytic effect of the barium and lanthanum or palladium in the over catalyst layer. In this instance, a significant feature is that, because the barium and lanthanum prevents thermal deterioration of the catalyst and works as a NOx absorbing agent to contribute the reduction of nitrogen oxides, the catalyst maintains a high NOx emission control efficiency even after the catalyst has been exposed to high temperature burned gases. Consequently, even when the over catalyst layer has the weight proportion less than {fraction (3/40)}, the catalyst exhibits the intended NOx emission control efficiency. If anything, the barium and lanthanum in the over catalyst layer which has rather a small weight proportion is more contributory to NOx emission control efficiency. In view of these facts, the over catalyst layer is more preferable to have the weight proportion in a range between {fraction (5/40)} and {fraction (16/40)}.
The catalyst may be modified with the result of exhibiting the intended performance in that the over catalyst layer contains barium and the under atalyst layer contains a catalytic metal. In this case, the utilization is made of a crystalline metal silicate as a support for the barium which prevents the barium from sulfur poisoning and/or water poisoning. Containing the barium in the over catalyst layer makes it easy to manufacture the catalyst. In the case where the catalyst is made through steps of washcoating a slurry of barium on a carrier and further washcoating a slurry of the metal silicate mixed with a catalytic metal over the barium contained coating, if the slurry of the metal silicate mixed with a catalytic metal is acid, there occurs the problem that the barium in the under catalyst layer eluates in the form of a barium hydroxide Ba(HO)2 into the slurry. Because the catalyst of the invention is, however, manufactured by a step of washcoating a slurry containing barium after having formed the under catalyst layer, the problem of the elusion of barium is not encountered.
The amount of barium in a range of 7 and 45% by weight of the total amount of the over and under catalyst layers is preferable for the catalyst to produce an improvement in NOx emission control efficiency. If the lower limit is exceeded, the barium is difficult to exhibit sufficiently its effect. On the other hand, if the upper limit is exceeded, the catalyst experiences a decline in NOx emission control efficiency this has been considered to be caused by an adverse effect of a large amount of the barium to the performance of other catalytic metals. In view of this point, an appropriate amount of barium is proved to be in a range between 10 and 30%.
The catalyst of the invention is manufactured in various manners. Specifically, when forming the under catalyst layer with barium contained, a slurry of alumina, ceria and binder, such as alumina binder, mixed with an appropriate amount of distilled water is washcoated on a monolith type honeycomb carrier. The coating is dried at a temperature between 150 and 300xc2x0 C. for two to four hours and burned in the air at a temperature of approximately 500xc2x0 C. for one to four hours. The barium in the form of a solid barium compound powder is contained in the slurry. As the solid barium compound, a barium oxide (BaO), a barium dioxide (BaO2) a barium carbonate (BaCO3) and a barium sulfate (BaSO4) can be employed. In order to support the palladium in the under catalyst layer, the coating after having been burned is impregnated with a palladium nitrate solution and dried and burned.
Alternatively, an under coating is prepared by washcoating a slurry of alumina, ceria and alumina binder mixed with distilled water on the honeycomb carrier and drying and burning the slurry. Thereafter, the coating is impregnated with a palladium solution and with a barium solution in this order or vise versa. A solution of barium and palladium may be admitted. As the barium solution, it is preferred to employ a barium acetate solution and a barium nitrate solution. The under catalyst layer may be achieved by a number of times of impregnation a coating with palladium and barium and drying the coating and a final burning treatment of the coating.
When forming the over catalyst layer, a slurry of a powdered zeolite (crystalline metal silicate) with platinum and rhodium supported, a ceria powder and binder mixed with distilled water is prepared. The slurry is washcoated over the under catalyst layer, dried at a temperature between 150 and 300xc2x0 C. for two to four hours and burned in the air at a temperature of approximately 500xc2x0 C. for one to four hours. The powdered zeolite with platinum and rhodium supported is prepared by producing a slurry of a mixture comprising a zeolite powder, a palladium solution and a rhodium solution and spraydrying and burning the slurry. Otherwise, the slurry may be solidified by evaporating solution liquids. Alternatively, the zeolite powder may be impregnated with a platinum solution and a rhodium solution, and dried and burned. A dinitrodiamine platinum solution and a rhodium nitrate solution may be employed as the platinum solution and rhodium solution, respectively.
Crystalline metal silicates are a porous material whose pore has a majority of microscopic pores and includes an aluminum group of metals as a main metal component of the crystal. Aluminosilicate silicate, i.e. zeolite, which is typical as an aluminum group metal, includes Y-type zeolite, moldenite, MFI-type zeolite, and xcex2-type zeolite. In place of aluminum or together with aluminum, metal silicates containing gallium (Ga), cerium (Ce), manganese (Mn) or terbium (Tb) may be employed.
The ceria as a cerium oxide may be added in various forms. For example, if the ceria is added into the over catalyst layer, the ceria may be mixed with the metal silicate as a support for the platinum and rhodium. Alternatively, the ceria with the platinum and rhodium supported thereby may be mixed with the metal silicate with the platinum and rhodium supported thereby. The same forms can be taken to form the under catalyst layer.
While the ceria is available as a cerium oxide, it is easy to experience thermal deterioration. In view of thermal resistance, a double oxide of cerium and zirconium (Zr) is preferable to be employed as a cerium oxide. Alumina may be added together with the cerium oxide.
When forming the over catalyst layer with barium contained, the under catalyst layer is formed in advance by washcoating a slurry of alumina, ceria and binder mixed with an appropriate amount of distilled water on a monolith type honeycomb carrier. The coating is dried at a temperature between 150 and 300xc2x0 C. for two to four hours and burned in the air at a temperature of approximately 500xc2x0 C. for one to four hours. The palladium is contained in the under catalyst layer by impregnating the coating with a palladium nitrate solution and drying and burning the coating.
Thereafter, the over catalyst layer is formed by washcoating a slurry of powdered zeolite with platinum and rhodium supported thereby, ceria and binder mixed with an appropriate amount of distilled water, drying the coating at a temperature between 150 and 300xc2x0 C. for two to four hours and burning it in the air at a temperature of approximately 500xc2x0 C. for one to four hours.
The impregnation with barium can be carried out in various forms. For instance, a slurry comprised of a mixture of the zeolite powder (not containing a catalytic metal) and solid barium particles, a palladium solution and a rhodium solution is spraydried and burned.
A solution of platinum, a solution of rhodium and a solution of barium may be added to the zeolite powder (not containing a catalytic metal). Otherwise, solid barium powder may be added into the slurry to be coated for the over catalyst layer. Alternatively, after impregnating with a barium solution a mixture of the powdered zeolite with platinum and rhodium supported thereby, powdered ceria and binder, a slurry may be prepared by adding distilled water to the mixture.
In order to add barium in both under and over catalyst layers, a slurry is prepared by mixing alumina, ceria and binder with distilled water and washcoated on a monolith type honeycomb carrier. The coating is dried at a temperature between 150 and 300xc2x0 C. for two to four hours and burned in the air at a temperature of approximately 500xc2x0 C. for one to four hours. Thereafter, the coating is impregnated with a palladium nitrate, dried at a temperature between 150 and 300xc2x0 C. for two to four hours and burned in the air at a temperature of approximately 500xc2x0 C. for one to four hours. Subsequently, a slurry is prepared by mixing powdered zeolite with platinum and rhodium supported thereby, ceria and binder with an appropriate amount of distilled water and washcoated over the under catalyst layer. The coating is dried at a temperature between 150 and 300xc2x0 C. for two to four hours and burned in the air at a temperature of approximately 500xc2x0 C. for one to four hours. Finally, the coatings are impregnated with a barium solution, achieving the under and over catalyst layers.
With regard to the under catalyst layer, the weight proportions of the alumina, ceria and binder relative to the honeycomb carrier are preferred to be 2-20%: less than 20%:1-10%, and more suitably to be 4-10% 1-10%:2-5%.
On the other hand, with regard to the over catalyst layer, the weight proportions of the zeolite, ceria and binder relative to the honeycomb carrier are preferred to be 6-35%: less than 35%:2-20%, and more suitably to be 5-25%:1-6%:5-10%.
The barium solution contains suitably barium between 0.5% by weight and a saturated concentration and more suitably higher than 1% by weight. The impregnation with the barium solution is suitably performed at a temperature between 10 and 40xc2x0 C.
As the alumina, xcex3-alumina containing an appropriate amount, for instance 7.5% by weight, of lanthanum is suitably employed.
The amount of palladium is preferred to be between 0.5 and 20 grams, and more suitably between 1 and 7 grams, per one liter of the volume of the honeycomb carrier.
The weight proportion between the platinum and rhodium is preferably 75:1, and the total amount of the platinum and rhodium is preferably in a range between 0.5 and 6 grams, and more suitably between 1 and 4 grams, per one liter of the volume of the honeycomb carrier.
The catalyst containing at least one of barium and lanthanum in the under catalyst layer, a water absorbing agent in the over catalyst layer, and a catalyst metal in at least one of the under and over catalyst layers exhibits significantly excellent performance of reducing nitrogen oxides (NOx), hydrocarbons (HC) and carbon monoxide (CO) in an automotive engine exhaust gas to nitrogen (N2), hydrogen dioxide (HO2) and carbon dioxide (CO2), respectively.